The core binding factors (CBFs) are a small family of transcription factors that consist of a DNA binding subunit encoded by the Runx1, Runx2, or Runx3 genes, and a common non-DNA binding CBF2 subunit encoded by the Cbfb gene. We showed that mice with reduced (15% of normal) CBF2 levels generate all blood cell lineages with the exception of T and NK cells. T cell development in CBF2-insufficient mice aborts at the specification stage, as the very earliest markers of T cell differentiation fail to be expressed. NK cell development also fails very early, at the transition between NK progenitors and immature NK cells. We propose to further characterize the NK cell defect caused by CBF2 insufficiency by determining which DNA-binding Runx subunit (or subunits) are required for the progression of NK progenitors to immature NK cells. We will also use in vitro assays of T and NK cell development to probe the CBF's biochemical functions in these lineages. Specifically, we will identify and characterize chromatin-remodeling proteins recruited by the CBFs to their target genes in T and NK cells, and assess the relevance of these proteins for T and NK cell development. We will assess chromatin occupancy by the CBF-interacting proteins to determine whether they associate with all or a subset of genes bound by the CBFs, and if their occupancy correlates with active or inactive chromatin states. Project Narrative: These studies aim to understand how proteins that are essential for lymphocyte formation perform their functions. This is an important area of research because disruption of lymphocyte development can lead to immune disorders, leukemia, and lymphoma.